Disc screens are utilized for screening or classifying discrete materials such as wood chips, municipal waste, and the like. Disc screens are commonly used in the paper industry to separate wood chips on the basis of thickness prior to pulping. Disc screens have a screen bed with a series of corotating, spaced, parallel shafts, each of which has a longitudinal series of concentric screen discs which are spaced from one another on the shafts. The screen discs of one shaft may interdigitate with the discs of the adjacent shafts. Spaces between the adjacent discs, defined as Interdisc Facial Opening (IFO) permit only material of a prescribed size or smaller to pass downward through the bed of rotating discs. Since the discs are all driven to rotate in a common direction from the in-feed end of the screen bed to the out-feed or discharge end of the bed, particles of material which are larger than the prescribed size will be advanced on the bed to the out-feed end of the bed. In some uses of disc screens, smaller than desirable size material is removed from the material flow, while in other uses larger than desirable material is removed from the flow.
Disc screens in which the screen discs are rigidly attached to a central shaft by methods such as welding are susceptible to damage when, in the course of normal usage, foreign objects such as slightly large chips, rocks, tramp metal or other objects enter the screen and lodge between the discs, becoming trapped. If the screen discs are not free to flex so that the oversize chips or other foreign material may be dislodged, breakage or permanent distortion of the screen disc is likely to result.
Disc screens have been constructed with resilient plastic spacers between the screen discs to permit a degree of flexing of the discs. Disc screens incorporating resilient spacers are disclosed in U.S. Pat. Nos. 4,653,648; 4,741,444; 4,972,959 and 4,972,960. The resilient spacers allow the discs to tilt out of the plane perpendicular to their rotation about the central shaft, thereby allowing the foreign object to be dislodged from the screen without damaging or permanently deforming it.
Polyurethane has been employed as a resilient spacer material, and possesses many advantageous physical properties for such use. However, long term "set", which is a permanent change in dimension caused by load, can result in IFO changes over time. These changes in dimension are proportional to the amount of material originally present.
To secure best performance of the disc screen, resilient spacers should be selected of consistent resilience to ensure the uniform deflection of the discs on the central shaft. Variations in flexibility may be present in spacers of identical dimensions due to inherent process limitations in the manufacture of the resilient plastic material, which results in the materials having varying resiliency. Slight variations in resiliency of different spacers are increasingly noticeable as the spacing between discs and, therefore, the thickness of the spacer increases. Furthermore, discs which are spaced a greater distance apart by resilient spacers must be spaced by spacers of reduced resiliency, if the same maximum deflection is to be maintained. Polyurethanes of reduced resiliency generally suffer greater effects of "set" than do the more resilient polyurethanes.
A disc screen assembly is needed that will be insensitive to commonly encountered resilient material property variations and which will permit easy adjustment of the Interdisc Facial Openings (IFO).